The developing multichip module packaging technology promises higher connectivity, faster electrical performance, more efficient thermal management and better reliability than was possible with earlier approaches to electronics packaging. Polyimide, with its low dielectric constant and superior planarization characteristics, is playing a key role in the metal/polyimide hybrid wafer scale integration technology. Polyimides are typically-used to provide a substrate for microelectronic elements in a multichip module. To achieve dense packing of microelectronic elements, the polyimide layers are usually designed to be planar and stackable.
Full utilization of this technology requires a comparable advance in the technology used to interconnect the microelectronic elements within the multichip module. Transmission of optical signals through optical waveguides is a possibility for such interconnections, but there have previously been significant obstacles to its utilization with multichip modules.
One obstacle has been the inability to obtain a difference between the indices of refraction of the waveguides and the surrounding medium large enough to prevent leakage of signals from the waveguides. Signal leakage is undesirable because of the potential for cross-talk between adjacent waveguides. Although the degree of cross-talk can be reduced by spacing the waveguides a sufficient distance apart, such a remedy is unsatisfactory where dense packing of components is desired. High waveguiding loss is a critical concern in multichip module applications, since interconnection lengths may exceed several centimeters.
Another obstacle has been the inability to form waveguides in polyimide that are sufficiently planar in vertical cross section so as not to interfere with the multilayer geometry of multichip module packaging. This packaging requires a planar structure so that multiple layers can be stacked on top of one another. Nonplanar ridge waveguides have been formed in polyimides, but they are not suitable for use in multichip module packaging.
A further obstacle has been inadequate thermal stability of waveguides when subjected to normal processing conditions. Since multichip module fabrication processes may involve temperatures exceeding 300.degree. C., waveguide characteristics of optical interconnections must not be adversely affected by high temperatures.
What is needed is a technique for forming waveguides in polyimide that show low loss characteristics and have a sufficiently large refractive index difference between the waveguide and the surrounding material, that are substantially planar so as to be compatible with the geometry of multichip module packaging, and that have sufficient thermal stability to withstand the high temperature used in multichip module fabrication processes.
The method of the present invention can provide waveguides that meet these needs. The method is further advantageous in that it is versatile, being useful for forming a variety of optical components, such as gratings, microlenses, microprisms, or the like, by a process that is economical and adaptable for use in conjunction with other fabrication processes commonly used in integrated optics or microelectronics.